In a semiconductor device manufacturing system, a serious problem arises in a semiconductor device manufacturing process if there occurs oil backflow from a pump into a process chamber of the semiconductor device manufacturing system. Accordingly, use has conventionally been made of a so-called dry pump, a mechanical booster pump, a turbomolecular pump, and the like in which there is no occurrence of contact between suction gas and oil.
With respect to these dry pump, mechanical booster pump, screw pump, etc., a problem exists that shaft seals are provided at both ends, i.e. on the suction side and the discharge side, and particularly a seal gas amount of the shaft seal on the suction side and a leakage amount from the seal cause a reduction in pumping speed, so that there is no alternative but to use such a pump that has an unnecessarily high pumping speed.
Further, since molecular weights of process gas, carrier gas, generated gas, and so on are broad, i.e. from 1 to one hundred and several tens, the current situation is on that the foregoing pumps are selectively used depending on their pumping characteristics for those various gases and their inherent pumping regions.
On the other hand, a problem exists that since the pumping speed is lowered depending on the kind of gas to be exhausted, a pump having a large pumping speed is inefficiently used. Further, with respect to general dry pumps and mechanical booster pumps, a problem exists that product is deposited inside the pump between an inlet port and an outlet port.
The present inventor has proposed a screw vacuum pump in Patent Document 1. The screw pump proposed in Patent Document 1 has a structure in which equal leads are provided on the suction side and the discharge side of unequal leads.
Patent Document 1    Japanese Unexamined Patent Application Publication (JP-A) No. 2004-263629